Disregard my previous post - I misread your explanation. I am guessing that the fact that your CEL comes on when the wastegate should be at its maximum opening rate points to an out of range error in its EGR mapping. Maybe another hole in your vacuum system somewhere that is keeping the vacuum pump from maintaining proper levels?

__________________
Scott C.

2006 E320 CDI (120k miles)
FOR SALE: 1998 E300 Turbo Diesel - Black w/Tan Leather - Euro delivery (236k miles)